1. Field of the Invention
The present invention relates to an image forming method depending on the electrophotographic process or the electrostatic recording process, and more particularly relates to a cleanerless image forming method which is capable of forming an image without the use of a cleaning device for cleaning excess toner remaining on the photosensitive drum after transferring the image.
2. Description of the Related Art
In general, an image forming method as shown cross sectionally in FIG. 1 is used, for example, in the device for imaging a latent image (image), such as the electrophotographic apparatus or the electrostatic recording apparatus. Usually, the apparatus is provided with a latent image holder, for example, a sensitive drum 1, an electrification device 2 arranged on the periphery of the sensitive drum 1, an exposure device 3, a developing device 4, a transfer device 5 and a cleaning device 6 having a cleaning blade 6a is used. Therefore, an electrostatic charge is applied by the electrification device 2 on the surface of the sensitive drum 1, and the selective exposure is carried out by the exposure device 3 in the electrification area to form a latent image in the apparatus mentioned above. And, the toner is selectively adhered to the formed latent image area (after the toner image is formed) by the developing device 4, then, the obtained toner image is transferred on a transfer paper 7 in the transfer device 5. The remaining toner adhering to the surface of the sensitive drum 1 is
removed by the cleaning blade 6a of the cleaning device 6.
However, in the apparatus described above, the cleaning device 6 is required to be separately provided to the position opposing to the developing device 4. Therefore, a limitation is placed on the arrangement of the electrification device 2, the exposure device 3, the transfer device 5 and other devices required for forming certain images. Further, the flexibility in the design of the devices is decreased. And, a surface of the sensitive drum 1 is worn by the cleaning blade 6a, which causes disadvantages, such as the deterioration of the characteristics and the decrease of the service life, during the cleaning operation. Further, an ozone product is generated during electric charging in the electrification device 2, thereby, a negative electrode organic photo conductor (referred to as "OPC", hereinafter) is deteriorated. Therefore, the surplus ozone products are required to be exhausted immediately. However, this causes a problem in that an exhaust path for the ozone products is inhibited by the cleaning device 6. Further, the toner retrieved in the cleaning device 6 is required to be suitably discarded. Therefore, problems are created in that the maintenance becomes complicated and the peripheral devices are possibly tarnished and stained.
In view of the problems mentioned above, a device having an exclusive cleaning device for carrying out developing during the first rotation of the sensitive drum 1 by means of the developing device and for cleaning during the second rotation by means of the same developing device 4, has been proposed, as described in Japanese Patent Application Laid-Opened Official Gazette SHO-62-211681. However, in the case mentioned above, since the developing step and the cleaning step are carried out separately, it is necessary that the sensitive drum 1 has a longer periphery than the image length to be formed. Therefore, not only must the sensitive drum have a large size, but also the whole device.
The image forming device (referred to as "cleanerless image forming device" hereinafter) for retrieving the remaining toner simultaneously with developing an image by means of the developing device without using the cleaning device has been known and is described, for example, in Japanese Patent Application Laid-Open No. 133573, 1984 and Japanese Patent Application Laid-Open No. 157661, 1984. In the official gazettes as described above, the basic conception of the cleanerless image forming device is disclosed, and the essence thereof can be summarized as described below. The main construction of the well-known reversal developing method is shown cross sectionally in FIG. 2. This reversal developing method is used in many cases in electrophotographic printers, such as a laser printer. In the reversal developing method, the toner particles 8 charged to have similar polarity as a latent image holder, for example, the sensitive drum 1, are adhered to a portion of the surface of the sensitive drum where an electric does not exist (or exists in a small amount). Herein, the toner particles 8 are not adhered to the portion where the electric charge exists. For achieving such selective adhesion of the toner, the voltage V.sub.b (.vertline.V.sub.1 .vertline.&lt;.vertline.V.sub.b .vertline.&lt;.vertline.V.sub.0 .vertline.) between the electric potential V.sub.0 of the charged portion and the electric potential V.sub.1 of non-charged portion on the surface of the sensitive drum 1 is applied to a toner holder 4a (developing agent holder) in the developing device 4. And the adhesion to the sensitive drum 1 is controlled by the electric field between the charged portion. Then, the toner 8 is adhered to the sensitive drum 1 by the electric field between the non-charged portion. The toner 8 adhered to the sensitive drum 1 is transferred to an image supporter 7 by means of the well-known transfer device 5. During the transfer step, not all of the toner is transferred, and the remained toner 8' remaining after transfer is present on the surface of the sensitive drum 1 in the image form. In the usual image forming device, for example, the electrophotographic device, the remaining toner 8' is retrieved by means of the cleaning device 6 shown by the dashed lines. Then, the electric charge on the surface of the sensitive drum 1 is removed by means of an electric removal lamp 9, and it is returned to the latent image forming step (a uniform charging device 2 step by the charging and an exposure step by the exposure device 3). In the cleanerless image forming device, the remaining toner 8' is transferred to the developing step without using the cleaner device 6 and is retrieved in the developing device 4 simultaneously with developing.
Since the remaining toner 8' existing in the charged portion (non-exposed part or non-imaged part) in the latent image formed by the exposure of the exposure device 3 is certainly charged to have the same polarity as the latent image by means of the electrification device 2, it is transferred to the toner holder 4a side by means of the electric field (electric field caused by the potential difference between V.sub.0 and V.sub.b) for controlling the transfer of the toner 8 from the toner holder 4a to the sensitive drum 1. Simultaneously, the remaining toner 8, existing in the non-charged portion (namely, the exposure part or the image part) is affected by the force from the toner holder 4a to the sensitive drum 1 to remain on the surface of the sensitive drum 1. The newly supplied toner particles 8 are transferred from the toner holder 4a to the non-charged portion, thereby, the cleaning is carried out simultaneously with the developing.
As described above, since the cleaning device 6 and the waste toner box are not required in the cleanerless recording device, the miniaturization and the simplification of the device can be facilitated. Therefore, the merits as described below can be obtained. Since the remaining toner 8, retrieved in the developing device 4 can be reused, it becomes economical because toner is not wasted. Since the sensitive drum 1 is not worn by the cleaning blade 6a, it has a longer service life of.
However, in the cleanerless image forming device, a ghost image is possibly caused by the following reasons.
First, in a high humidity environment, since the paper as the image supporter 7 takes the moisture to be low resistance, the transfer efficiency becomes lower. Therefore, a lot of toner particles tend to remain on the surface of the sensitive drum 1. When the amount of the remaining toner 8' becomes excessive, it cannot be completely cleaned up in the developing device 4. Therefore, the remaining toner 8' stays on the non-imaged part to cause a positive ghost on a white portion of the transfer image (referred to as "positive ghost" image or "positive memory" hereinafter).
Second, when the amount of the remaining toner 8' becomes excessive, the light beam 3 is intercepted by the remaining toner 8' during the exposure step by the exposure device thereby, the damping of the electric potential on the surface of the sensitive drum 1 results in an insufficient the electric potential (referred to as "V.sub.1 '") in the intermediate between V.sub.0 and V.sub.1. In the portion as described above, the developing voltage becomes as V.sub.b -V.sub.1 ', which is smaller than the developing voltage V.sub.b -V.sub.1 of the periphery exposure part. Therefore, the toner transfer amount from the toner holder 4a to the Sensitive drum 1 becomes smaller as compared with the periphery, thereby, the remaining toner image appears on the developing part of the transfer image as a void image (referred to as "negative ghost" image or "negative memory", hereinafter). This phenomenon notably appears especially in the half-tone image formed of the aggregation of the net point image and the line image, etc.
Japanese Patent Application Laid-Open No. 203183, 1987, discloses to removing the ghost image by applying a voltage to an electroconductive brush 10 having such formation as shown cross sectionally in FIG. 3 to be contacted slightly with the sensitive drum 1. Namely, the voltage having the reverse polarity to the electro static charge of the toner is applied to the electroconductive brush 10 by the direct current power, and the remaining toner 8' is absorbed at once by the brush 10 by the Coulomb force. Therefore, the amount of the remaining toner 8' on the surface of the sensitive drum 1 can be remarkably decreased, and the above mentioned ghost image can be avoided.
However, in the case of the above system, under the experiment by the present inventor, the deterioration of the cleaning characteristics is often found according to the amount of one component developing agent layer formed on the developing agent holder 4a and other developing conditions when the development cleaning is carried out on using one component developing agent. Further, it becomes clear that the sufficient condition carrying out only the developing cannot be always applied thereto. Namely, the paper as the image holder 7 holds (absorbs) much moisture under high humidity conditions, therefore, the resistance ratio is remarkably decreased. As a result, the electric charge provided from the transfer device 5 to the paper 7 moves to the thickness direction of the paper 7 to reach the toner particles on the surface of the sensitive drum 1, thereby, the toner is charged in the reverse polarity to the essential electro static charge. Since the toner charged in the reverse polarity is affected by the repulsive force caused by the electric field even if contacting with the electroconductive brush 10, it is not absorbed by the electroconductive brush 10. Further, the dispersion of the remaining toner image 8' can be kept substantially in it's original condition after passing through the brush. Therefore, the above mentioned ghost image cannot be avoided in such case.
Further, since the amount of the toner which can be held in the electroconductive brush 10 has limitations, the toner is naturally expelled to the surface of the sensitive drum 1 when it builds up to a specific level. The expelled toner is not dispersed in an image form like the remaining toner 8' and it shows the remarkably uniform dispersion, therefore, the above mentioned ghost image is not induced. However, in the case in which the solid image is sequentially output (sequential development of the solid image), a lot of the toner is held in the electroconductive brush 10 to cause the possible expulsion of the toner to the surface of the sensitive drum 1. In such case, the above mentioned ghost image is generated.
Because of the problems as mentioned above, the image forming by the conventional cleanerless image forming method is hard to carry out in a high humidity environment. Further, it has been caused a disadvantage that the property of the image capable of being output has the limitation.
Therefore, the first object of the present invention is to provide an image forming method which is capable of obtaining always producing a satisfactory image without ghost images and fogging by carrying out certain developing using one component toner (developing agent) and simultaneously cleaning up efficiently the remaining and adhered toner on the surface of the sensitive drum.
Further, another object of the present invention is to provide a cleanerless image forming device which is capable of producing a satisfactory image in a high humidity environment and capable of outputting any kind of image.